Nanocrystal surface-emitting lasers

ABSTRACT

An all-epitaxial, electrically injected surface-emitting green laser operates in a range of about 520-560 nanometers (nm). At 523 nm, for example, the device exhibits a threshold current density of approximately 0.4 kilo-amperes per square centimeter (kA/cm2), which is over one order of magnitude lower than that of previously reported blue laser diodes.

RELATED U.S. APPLICATION

This application claims priority to the U.S. Provisional Applicationentitled “Electrically Pumped Surface-Emitting Semiconductor GreenLaser,” by Yong-Ho Ra et al., Ser. No. 62/915,432, filed Oct. 15, 2019,hereby incorporated by reference in its entirety.

STATEMENT PER 35 U.S.C. 202(c)(6)

This invention was made with government support under ECCS1709207awarded by the National Science Foundation, and under W911NF-17-1-0388awarded by the U.S. Army/Army Research Office. The government hascertain rights in the invention.

BACKGROUND

Vertical cavity surface-emitting laser (VCSEL) diodes, first presentedin 1979, emit a coherent optical beam vertically from the device topsurface and offer a number of advantages compared to conventionaledge-emitting lasers, including lower threshold currents, circular andlow-divergence output beams, single longitudinal mode emission, longerlifetime, and easier production of dense two-dimensional arrays.Commercial VCSELs are fabricated on gallium arsenide (GaAs) and indiumphosphorus (InP) substrates, which emit light mostly in thenear-infrared wavelengths. Semiconductors based on gallium nitride (GaN)are the choice for lasers operating in the visible and ultravioletspectral ranges, and significant efforts have been devoted to developingGaN-based VCSELs. However, operating wavelengths have been largelylimited to the blue spectral range.

Dual dielectric distributed Bragg reflectors (DBRs) are an essentialcomponent of conventional VCSELs. DBRs consist of multiple alternativelayers of materials with a relatively large difference in refractiveindex to provide very high reflectivity. DBRs with nearlylattice-matched layers can be formed in GaAs-based and InP-basedsystems, but have remained a critical challenge for GaN-based systems.The large lattice mismatch between GaN and AlN (about 2.5 percent) andbetween GaN and InN (about 11 percent), together with the difficulty inachieving efficient p-type conduction, leads to GaN-based DBRs with highelectrical resistivity, large densities of defects and dislocations, andrelatively low reflectivity. In addition, the presence of a strongpolarization field and the resulting quantum-confined Stark effect(QCSE) of conventional c-plane GaN devices further reduce the rate ofradiative recombination, resulting in higher thresholds and unstableoperation. To address these issues, GaN-based blue VCSELs have beenreported by utilizing AlInN/GaN DBRs or dual-dielectric DBRs, and bygrowing the devices on an m-plane GaN substrate. The resulting devices,however, still exhibit very large threshold current density (J_(th)greater than ten kilo-amperes per square centimeter) at roomtemperature, with operating wavelengths still limited to 400-460nanometers (the blue spectral range).

To date, there has not been a demonstration of all-epitaxialsurface-emitting laser diodes operating in the green wavelength range,to which human eyes are most sensitive. A previously reportedroom-temperature continuous wave (RTCW) surface-emitting green laserdiode relied on the use of DBRs and wafer bonding to a copper plate forlow thermal resistance. The realization of a low current threshold, highefficiency, all-epitaxial surface-emitting green laser diode will enablemany exciting applications including projection displays such aspico-projectors, plastic optical fiber communication, wirelesscommunication, optical storage, smart lighting, and biosensors.Surface-emitting semiconductor lasers have been widely used in datacommunications, sensing, facial identification, and augmented realityglasses.

SUMMARY

Disclosed herein are all-epitaxial, electrically injected nanocrystalsurface-emitting lasers (NCSELs) that do not necessarily use distributedBragg reflectors (DBRs). In embodiments, the devices operate in a rangeof about 520-560 nanometers (nm), the green wavelength range. At 523 nm,for example, the devices exhibit a threshold current density J_(th) ofapproximately 0.4 kilo-amperes per square centimeter (kA/cm²), which isover one order of magnitude lower compared to that of previouslyreported blue laser diodes. Consequently, low threshold surface-emittinglaser diodes can operate at wavelengths from the ultraviolet to the deepvisible (about 200-600 nm), and device performance is no longer limitedby the lack of high quality DBRs, large lattice mismatch, and substrateavailability. The NCSELs disclosed herein can be used to achieveultra-small semiconductor lasers that can improve the efficiency andresolution of displays by ten to 100 times.

These and other objects and advantages of the various embodiments of thepresent invention will be recognized by those of ordinary skill in theart after reading the following detailed description of the embodimentsthat are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification and in which like numerals depict like elements,illustrate embodiments of the present disclosure and, together with thedetailed description, serve to explain the principles of the disclosure.The drawings are not necessarily to scale.

FIG. 1A illustrates a monolithic device in embodiments according to thepresent invention.

FIG. 1B is a top-down view of a nanocrystal array in embodimentsaccording to the present invention.

FIG. 1C illustrates a structure of a nanowire in embodiments accordingto the present invention.

FIG. 1D illustrates the reciprocal lattice of a photonic nanocrystalstructure in embodiments according to the present invention.

FIG. 1E illustrates the photonic band structure of a nanocrystalsurface-emitting laser (NCSEL) in embodiments according to the presentinvention.

FIG. 1F shows the photoluminescence (PL) spectrum measured at roomtemperature of a nanocrystal or nanowire in embodiments according to thepresent invention.

FIG. 1G shows the PL emission spectra of a core-shell multi-quantum disknanocrystal in embodiments according to the present invention and of aconventional multi-quantum disk nanocrystal.

FIG. 2A shows the high-angle annular dark-field (HAADF) atomic-numbercontrast image of a representative nanocrystal in embodiments accordingto the present invention.

FIG. 2B is a high-magnification image of a region of the image in FIG.2A.

FIG. 2C shows the high-magnification HAADF image of the active region ofa nanocrystal in embodiments according to the present invention.

FIG. 3A illustrates a NCSEL in embodiments according to the presentinvention.

FIG. 3B shows an example of a current-voltage curve of the NCSEL of FIG.3A.

FIG. 3C shows the electroluminescence (EL) spectra of the NCSEL of FIG.3A.

FIG. 3D shows variations in the output power versus injection currentfor the NCSEL of FIG. 3A.

FIGS. 3E and 3F illustrate variations of spectral linewidth andwavelength peak position, respectively, under different injectioncurrent densities for the NCSEL of FIG. 3A.

FIG. 4A shows the EL spectra for different polarizations for a NCSEL inembodiments according to the present invention.

FIG. 4B illustrates variations of EL intensity versus polarization anglefor a NCSEL in embodiments according to the present invention.

FIG. 5 shows the variation of the effective refractive index for a NCSELin embodiments according to the present invention.

FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 7A, and 7B illustrate an example of stepsused to fabricate an NCSEL in embodiments according to the presentinvention.

FIG. 8 illustrates a nanocrystal structure in embodiments according tothe present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. While described in conjunction with theseembodiments, it will be understood that they are not intended to limitthe disclosure to these embodiments. On the contrary, the disclosure isintended to cover alternatives, modifications and equivalents, which maybe included within the spirit and scope of the disclosure as defined bythe appended claims. Furthermore, in the following detailed descriptionof the present disclosure, numerous specific details are set forth inorder to provide a thorough understanding of the present disclosure.However, it will be understood that the present disclosure may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail so as not to unnecessarily obscure aspects of the presentdisclosure.

The figures are not necessarily drawn to scale, and only portions of thedevices and structures depicted, as well as the various layers that formthose structures, are shown. For simplicity of discussion andillustration, only one or two devices or structures may be described,although in actuality more than one or two devices or structures may bepresent or formed. Also, while certain elements, components, and layersare discussed, embodiments according to the invention are not limited tothose elements, components, and layers. For example, there may be otherelements, components, layers, and the like in addition to thosediscussed.

Some portions of the detailed descriptions that follow are presented interms of procedures and other representations of operations forfabricating devices like those disclosed herein. These descriptions andrepresentations are the means used by those skilled in the art of devicefabrication to most effectively convey the substance of their work toothers skilled in the art. In the present application, a procedure,operation, or the like, is conceived to be a self-consistent sequence ofsteps or instructions leading to a desired result. operations describedas separate blocks may be combined and performed in the same processstep (that is, in the same time interval, after the preceding processstep and before the next process step). Also, the operations may beperformed in a different order than the order in which they aredescribed below. Furthermore, fabrication processes and steps may beperformed along with the processes and steps discussed herein; that is,there may be a number of process steps before, in between, and/or afterthe steps shown and described herein. Importantly, embodiments accordingto the present invention can be implemented in conjunction with theseother (perhaps conventional) processes and steps without significantlyperturbing them. Generally speaking, embodiments according to thepresent invention can replace portions of a conventional process withoutsignificantly affecting peripheral processes and steps.

The present disclosure introduces a nanocrystal surface-emitting laser(NCSEL) diode that does not necessarily include distributed Braggreflectors (DBRs) or the like (e.g., Bragg mirrors or dielectricmirrors), and that can operate efficiently in the green spectrum.

FIG. 1A illustrates a monolithic device 100 that includes a nanocrystalarray 102 on a substrate 106 in embodiments according to the presentinvention. A monolithic device is a device that has a single substrate;that is, the nanocrystal array 102 is fabricated on a single substrate,and the nanocrystal array is not separated from that substrate whenincorporated into, for example, a laser.

The nanocrystal array 102 includes a number of nanocrystals, ornanowires, 104 (each nanowire is a nanocrystal, and the array ofnanocrystals includes an array of nanowires, and so these terms may beused interchangeably herein). FIG. 1B is a top-down view of the array102 in embodiments according to the present invention. The size,spacing, and surface morphology of the nanowires 104 in the array 102are precisely controlled. Each of the nanowires 104 has a hexagonalshape; that is, they each have a transverse cross-section that ishexagonal. The array 102 includes multiple rows of nanowires, with eachrow including multiple nanowires. In an embodiment, the nanowires 104 inthe array 102 are arranged in a triangular lattice. The diameter andlattice constant of the nanowires 104 are denoted as d and a,respectively, in FIG. 1B. In an embodiment, the nanowires 104 have aspacing (gap) of approximately 30 nanometers (nm) between neighboringnanowires, a height of approximately 600 nm, a lateral size ofapproximately 230 nm, and the lattice constant (pitch) is approximately250 nm. The nanowires 104 exhibit uniform length, smooth sidewalls, andhigh (depth-to-width) aspect ratio. Due to the efficient strainrelaxation, the nanostructures of the nanowires 104 are free ofdislocations.

FIG. 1C illustrates the structure of a nanowire 104 in embodimentsaccording to the present invention. In embodiments, the nanowire 104includes an n-doped gallium nitride (n-GaN) cladding layer or region110, a heterostructure or active region 112, and a p-doped GaN (p-GaN)cladding layer or region 114. (The active region 112 includes thedisclosed heterostructure, and these terms may be used interchangeablyherein.) In embodiments, the active region 112 is an indium-GaN andaluminum-GaN (InGaN/AlGaN) core-shell heterostructure that includesmultiple InGaN quantum disks 122 in the active region. In embodiments,the n-type GaN cladding layer 110 is approximately 370 nm thick(measured in the longitudinal direction of the nanowire 104), and thep-type GaN cladding layer 114 is approximately 190 nm thick; however,the invention is not so limited.

FIG. 1C also illustrates an expanded view of the heterostructure (activeregion) 112 in embodiments according to the present invention. Thequantum disks 122 are incorporated on semipolar planes of the activeregion 112, which can significantly reduce the quantum-confined Starkeffect (QCSE). A unique AlGaN shell structure 124 is also disposed on asemipolar plane surrounding the quantum disks 122 as shown in FIG. 1C.The shell structure 124 suppresses surface recombination.

Because the quantum disks 122 and the shell structure 124 are disposedon semipolar planes of the nanowire 104, the heterostructure 112 isgenerally cone-shaped or pyramid-shaped, and is narrower as distancefrom the substrate 106 (FIG. 1A) increases. The unique three-dimensional(3D) structure of the active region 112 provides an emission area thatis two times larger than that of conventional quantum disk/dot activeregions aligned vertically in horizontal planes.

In embodiments, the shell structure 124 is interleaved with the quantumdisks 122. That is, in embodiments, the shell structure 124 includesmultiple layers, and each shell layer (except for the outermost shelllayer) is disposed between two quantum disks (and as such, each quantumdisk is disposed between two shell layers).

The shell structure 124 spontaneously forms on the sidewalls of theactive region 112 during fabrication (see the discussion accompanyingFIG. 6A, below). This results in drastically reduced non-radiativesurface recombination due to the effective lateral confinement offeredby the large band-gap AlGaN shell 124. Moreover, the unique quasi-3Dstructure of the active region 112 exhibits massively enhanced surfaceemission and improved carrier injection efficiency, due to the muchlarger surfaces of the active area. Such a semipolar structure caneffectively suppress QCSE due to the reduced polarization fields.

The nanowires 104 emit light (laser light, e.g., light by stimulatedemission or population inversion) having wavelengths in a range of520-560 nanometers (the green spectral range) at a threshold currentdensity that is at least an order of magnitude less than ten kiloamperesper square centimeter (kA/cm²). In embodiments, the nanowires 104operate at 523.1 nm at a threshold current density on the order ofapproximately 400 A/cm² and less, and exhibit highly stable operation atroom temperatures.

In an embodiment, the substrate 106 (FIG. 1A) is light-transmissive;that is, light produced by the nanocrystal array 102 can pass throughthe substrate. In another embodiment, the substrate is notlight-transmissive (e.g., it is opaque).

The nanocrystal array 102 exhibits a photonic band-edge resonant effect.The nanocrystal array 102 can be utilized to produce electricallyinjected nanocrystal surface-emitting lasers, referred to herein asNCSELs. In particular, the nanocrystal array 102 can be utilized toproduce NCSELs that operate in the green spectral range, although theinvention is not so limited. Embodiments according to present inventionintroduce a viable approach that can achieve high performancesurface-emitting laser diodes from the deep ultraviolet (UV) range tothe deep visible range (e.g., approximately 200-600 nm). Significantlyand advantageously, such laser diodes are achieved without necessarilyusing conventional thick and high-resistivity DBRs or the like. Theachievement of coherent lasing oscillation is confirmed by the far-fieldemission pattern and by detailed polarization measurements.

Design and simulation of the NCSELs disclosed herein, including energyband diagram and mode profile, were performed using a two-dimensionalfinite-element method (2D-FEM) simulation with Maxwell's equations (seealso FIG. 5 and the accompanying discussion, below). FIG. 1D illustratesthe reciprocal lattice of a photonic nanocrystal structure inembodiments according to the present invention. The reciprocal latticeof the photonic crystal structure has six equivalent gamma (Γ′) pointsin the Brillouin zone, which are coupled together by the Bragg gratingvectors (e.g., K₁ and K₂). The corresponding wavelength of light forms astanding wave resonant in the photonic crystal without a DBR (e.g.,dielectric mirror). In addition to such in-plane coupling, there is alsoout-of-plane coupling between the six Γ′ points and the Γ point that hasa zero in-plane wavevector. The wavevector is essentially vertical,thereby leading to surface emission.

FIG. 1E illustrates the photonic band structure of NCSELs disclosedherein, calculated from a 2D-FEM simulation for transverse magnetic (TM)polarization (E ∥c-axis). The Γ point in the fourth band is located atapproximately 0.48 a/λ (where a is the lattice constant, and λ iswavelength), which corresponds to a wavelength of about 520 nm. Thegroup velocity is determined by the slope of the dispersion curve in thephotonic band structure. At the band edge, the low group velocity isachieved when the slope of dispersion curve become zero (e.g., near theΓ point, the group velocity of light becomes zero (dw/dk→0)), therebyleading to the formation of a stable and large single-cavity mode. Themode intensity is mostly distributed in the nanocrystals (the nanowires104 of FIG. 1C). The extremely low group velocity leads to a longinteraction time between radiation field and active material, andconsequently gives rise to a strong gain enhancement. Photons are alsoconfined around the active region 112 in the vertical direction, due tothe higher average refractive index in the active region. The modeintensity profile in the vertical direction is shown in FIG. 5.

FIG. 5 shows the variation of the effective refractive index along thegrowth direction and the TM polarized mode intensity profile for NCSELsin embodiments according to the present invention. Due to largerrefractive index for InGaN compared to GaN, the effective refractiveindex is higher around the active region. The effective refractive indexis calculating assuming the nanocrystals have a flat top morphology, thelattice constant is 250 nm, the spacing between nanocrystals is 30 nm,and the refractive index is 1.75 for the filling material (polyimide),2.69 for InGaN, 2.35 for AlGaN, and 2.38 for n-GaN and p-GaN. Thediameter of the nanocrystals is approximated to grow from 170 nm to 255nm within a height of 50 nm. Due to the high viscosity of the fillingmaterial and the small spacing between nanocrystals, air gaps exist nearthe root of the GaN nanocrystals. Considering the lengths of thesegments in FIG. 1A, the variation of the effective refractive index inthe vertical direction and the corresponding TM polarized mode arecalculated as shown in FIG. 5. The mode is mostly confined near theactive region 112 (FIG. 1C) due to the higher refractive index.

The realization of NCSELs as disclosed herein requires a precise controlof the nanocrystal (nanowire) size, spacing, and uniformity across arelatively large area. The fabrication of the nanocrystal array 102(FIG. 1A) is achieved by a special technique of selective area epitaxyusing plasma-assisted molecular beam epitaxy (MBE) (see also FIGS. 7Aand 7B and the accompanying discussion, below).

FIG. 1F shows the photoluminescence spectrum measured at roomtemperature of a nanocrystal or nanowire 104 (FIG. 1C) in embodimentsaccording to the present invention. The peak emission wavelength isapproximately 523 nm. Significantly enhanced photoluminescence (PL)emission intensity was measured for the InGaN/AlGaN core-shell structuredisclosed herein, compared to similar structures but without an AlGaNshell.

FIG. 1G shows the PL emission spectra of an InGaN/AlGaN core-shellmulti-quantum disk nanocrystal 104 (FIG. 1C) and of a conventionalInGaN/GaN multi-quantum disk nanocrystal without an AlGaN shell, inembodiments according to the present invention. The PL spectra shown inFIG. 1G is measured at room temperature (300 degrees Kelvin (° K)) usinga 405 nm laser as the excitation source. The PL intensity of thesemipolar InGaN/AlGaN core-shell 112 (FIG. 1C) is enhanced by nearly afactor of eight compared to the conventional InGaN/GaN heterostructurewithout the AlGaN shell.

FIG. 2A shows the high-angle annular dark-field (HAADF) atomic-numbercontrast image of a representative InGaN nanocrystal (nanowire) 104 inembodiments according to the present invention. The InGaN/AlGaNheterostructure 112 is formed as a cone or in a cone-like shape. This isdue to the formation of n-GaN nanocrystals that have a Ga-polarity andpyramid-like morphology as described above. The resulting uniquestructure takes advantage of the semipolar effect in the active region112 to reduce the polarization field in GaN wurtzite structures.

FIG. 2B is a high-magnification image taken from the marked region inFIG. 2A. The sloping, multiple quantum disk layers 112 can be moreclearly observed in FIG. 2B. The formation of multiple quantum diskheterostructures on semipolar planes of [0113] orientation is furthersupported by a representative selective area electron diffraction (SAED)pattern analysis.

Due to the quasi-3D structure of the InGaN/AlGaN multi-quantum disklayers 112, the transmission electron microscopy (TEM) images havedifferent projection effects from the different layers. From thewide-thickness layers with the bright contrast in the middle region ofFIG. 2B, it can be seen that multiple quantum disk and shell structuresare formed in a cone-like structure as described above. The formation ofsuch a cone-shaped active layer is further supported by the formation ofthe n-GaN structure as described above. The unique 3D structure providesan emission area that is two times larger than that of the typicalquantum disk/dot active regions aligned vertically with the samediameter.

FIG. 2C shows the high-magnification HAADF image of InGaN/AlGaN activeregion 112 (FIG. 2A) in embodiments according to the present invention.The presence of highly uniform multiple quantum disk layers can beclearly identified by different levels of contrast. No noticeableextended misfit dislocations or stacking faults are observed. Thethicknesses of the InGaN disks 122 and AlGaN barriers 124 of FIG. 2C areapproximately 2.5 nm and approximately 1.5 nm, respectively; however,the invention is not so limited.

To further confirm elemental distribution of the active region 112, anenergy dispersive x-ray spectrometry (EDXS) analysis was performed alongthe growth direction of the InGaN/AlGaN heterostructure 112, which islabeled as “1” in FIG. 2C. Because the atomic number of Al is smallerthan the atomic number of In, it has a darker contrast in the TEM image.The In signal exhibits a maximum in the brighter regions and drops inthe darker regions. In contrast, the Al signal shows clear peaks in thedarker regions, confirming the formation of InGaN/AlGaN quantum disk andshell heterostructures.

The presence of the Al-rich AlGaN shell layer 124 formed spontaneouslyon the sidewall(s) of the InGaN quantum disk(s) 122 is also confirmed byEDXS point analysis. The spontaneously formed AlGaN shell structure 124can effectively suppress nonradiative surface recombination, a primarylimiting factor for the performance of conventional nanostructuredevices.

Moreover, the semipolar InGaN/AlGaN core-shell heterostructure 112offers several distinct advantages, including significantly reducedpolarization fields and enhanced light emission efficiency, as well assignificantly improved carrier injection efficiency and luminescenceefficiency, compared to conventional quantum disk/dot structures (seeFIG. 1G and the accompanying discussion). Such a unique structure cannotbe fabricated by conventional top-down approaches because the activeregion is predefined by the film structure.

FIG. 3A illustrates a NCSEL 300 that includes the monolithic device 100,as well as other components (see FIG. 6F and the accompanyingdiscussion, for example), in embodiments according to the presentinvention. The NCSEL 300 can be fabricated using planarization,polyimide passivation, contact metallization, and photolithographytechniques as described below in conjunction with FIGS. 6A-6E. The NCSEL300 emits laser light in the vertical direction (in the orientation ofFIG. 3A), away from the substrate 106 as shown in the figure.

FIG. 3B shows an example of a current-voltage (I-V) curve of the NCSEL300 with an example surface area of approximately 25 square micrometers(μm²), in embodiments according to the present invention. The I-V curveshows rectification characteristics with a sharp turn-on voltage ofapproximately 3.3 V at room temperature. The leakage current isnegligible under reverse bias, as shown in the inset of FIG. 3B, whichshows current density versus voltage in embodiments according to thepresent invention. The NCSEL 300 exhibits excellent I-V characteristics,which is partly due to the significantly reduced defect density andenhanced dopant incorporation in the nanocrystals 104 of the device. Theemitted light is emitted from the top surfaces of the nanocrystals(nanowires) 104 (FIG. 1A).

FIG. 3C shows the electroluminescence (EL) spectra of the NCSEL 300measured under different injection currents, in embodiments according tothe present invention. At a low injection current density ofapproximately 200 A/cm², the device exhibits a broad emission spectrumcentered at a wavelength of approximately 524 nm, with afull-width-at-half-maximum (FWHM) of approximately 30 nm, whichcorresponds to the spontaneous emission of the active region 112 (FIG.1C). A sharp lasing peak at a wavelength of approximately 523.1 nm isobserved with increasing injection current, with a narrow linewidth ofapproximately 0.8 nm. The strong lasing spot is shown in the inset ofFIG. 3A, recorded at a current density approximately one kA/cm².Variations of the output power versus injection current exhibit a clearthreshold at approximately 400 A/cm², as shown in FIG. 3D. The measuredlasing threshold is significantly lower compared to conventionalGaN-based vertical cavity surface-emitting lasers (VCSELs). An outputpower of approximately 12 micro-watts (μW) was measured at an injectioncurrent density of approximately one kA/cm² under continuous waveoperation. The output power shows saturation with increasing injectioncurrent, due to heating effects. The measured output power issignificantly higher than previously reported values of GaN-based VCSELsoperating at 460 nm and 500 nm.

FIGS. 3E and 3F illustrate variations of spectral linewidth (FWHM) andwavelength peak position, respectively, under different injectioncurrent densities, in embodiments according to the present invention. Inthese examples, the spectral linewidth decreases from approximately 30nm to 0.8 nm at the threshold current density. It is also seen that thelasing peak position stays nearly constant at approximately 523 nm abovethat threshold, suggesting highly stable lasing of the NCSELs disclosedherein. The low threshold current density and highly stable emission aredirectly related to the robust photonic band edge mode of thenanocrystal optical cavity, the dislocation-free bottom-up nanocrystalstructure, and the reduced nonradiative surface recombination andsuppressed polarization-field with the extended emission area in theInGaN/AlGaN cone-like shell active region 112 (FIG. 1C).

The far-field radiation pattern of the nanocrystal laser structuredisclosed herein was simulated using a 3D finite-difference time-domain(FDTD) method. Because the wavelength near the Γ point has a very smallin-plane wavevector component, the wavevector is expected to be almostvertical. Due to that unique property of the Γ point, the far fieldpattern indeed exhibits a spot in the center with a very smalldivergence angle, which corresponds to highly collimated verticalemission.

The far-field patterns were measured with current densities below andabove the threshold of the NCSELs disclosed herein. Below the threshold,the far-field image shows nearly uniform emission without anyinterference fringes, suggesting the formation of a highly uniform bandedge mode in the nanocrystal array. When the current density is abovethe threshold, the lasing emission shows the presence of interferencefringes that indicate coherent emission. Such results provide strongevidence that the coherent lasing oscillation has been achieved in thenanocrystal arrays disclosed herein.

The polarization properties in the far-field of light emission of NCSELsdisclosed herein was studied. The degree of polarization is defined asρ=(I_(0°)−I_(90°))/(I_(0°)+I_(90°)), where I_(0°) and I_(90°) are the ELemission intensity corresponding to the electric field along zerodegrees (0°) and 90° direction, respectively. FIG. 4A shows the ELspectra measured under a current density of one kA/cm² for differentpolarizations for embodiments according to the present invention. The ELemission is highly polarized. The degree of polarization is as large as0.86. Variations of EL intensity versus polarization angle (θ) arefurther plotted in FIG. 4B for embodiments according to the presentinvention, showing a high degree of polarization at θ=0°. This is aremarkably stable and directional polarized emission compared toconventional photonic crystal laser devices. These studies provideunambiguous evidence for the achievement of a surface-emitting laser.

FIGS. 6A-6F illustrate an example of steps used to fabricate an NCSEL600 that includes the monolithic device 100 of FIG. 1A in embodimentsaccording to the present invention. The NCSEL 600 is an example of theNCSEL 300 previously discussed herein (see FIGS. 3A-3F and theaccompanying discussion). Where certain materials (elements),dimensions, and values are included in the discussion below, the presentinvention is not so limited.

In FIG. 6A, the nanocrystal array 102 is grown on the substrate 106. Inembodiments, each nanowire or nanocrystal 104 in the nanocrystal array102 includes an n-GaN cladding layer or region 110, a heterostructure oractive region 112, and a p-GaN cladding layer or region 114 aspreviously described herein (e.g., FIG. 1C).

More specifically, with reference to FIG. 7A, a patterned thin filmnano-hole mask 702 is fabricated on an n-type GaN template substrate 704on the substrate 106. In embodiments, the mask 702 is titanium (Ti), andthe substrate 106 is aluminum oxide (Al₂O₃). In embodiments, the n-typeGaN template substrate 704 is four microns (μm) thick, and the mask 702is ten nm thick. The mask 702 can be deposited on the substrate 704 byan e-beam evaporator system.

In FIG. 7B, a positive poly(methyl methacrylate) (PMMA) is used as aresist layer with a spin-coating technique. An e-beam lithographyprocess is utilized to obtain a uniform array of nano-holes 710. Inembodiments, then nano-holes 710 each have a diameter of approximately180 nm and a lattice constant of 250 nm arrayed in triangular lattices;however, the invention is not so limited. The lateral growth effect istaken into account in the pattern design. Subsequently, a methylisobutyl ketone:isopropyl alcohol solution may be used for thedevelopment process. Thereafter, the exposed area of the mask 702 can beetched down by a reactive ion dry-etching technique. The nano-holepatterned substrate 710 can then be cleaned and loaded into amolecular-beam epitaxy (MBE) growth chamber. Prior to selected areaepitaxial growth (SAG) of the GaN nanocrystals, a nitridation step canbe performed; consequently, in an embodiment, the mask 702 is convertedto a TiN film, which prevents the formation of cracks and degradation atelevated temperatures. In embodiments, the nanocrystal array 102 isgrown on the n-type GaN template 704 by radio frequency (RF)plasma-assisted MBE.

With reference again to FIG. 6A, and also to FIGS. 1C and 2A, theInGaN/AlGaN core-shell heterostructures 112 are formed (disposed) in thenanowires 104. First, an InGaN core 122 disk layer is grown on the topsurface region of n-GaN nanocrystal 110. Due to the strain-inducedself-organization effect, the size of the InGaN disk becomes smallerthan the n-GaN nanocrystal diameter. The incorporation of AlGaN barrierlayers, instead of GaN barrier layers, leads to the formation of anAlGaN shell structure surrounding the InGaN quantum disk active region,due to the smaller Al adatom diffusion length compared to Ga and Inadatom diffusion. As a consequence, the growth fronts including the topand sidewalls of the InGaN region can be covered by AlGaN layers,thereby leading to the spontaneous formation of large band-gap AlGaNshell structures 124. By repeating the growth process, coaxially alignedcone-like AlGaN shell layers 124 can be fabricated surrounding the InGaNmultiple quantum disk structures 122.

A polarity generally refers to the direction in which GaN growsconsidering Ga-N bonds that are collinear with the c-axis of thewurtzite crystal structure. The vector from Ga to N-direction defines[0001], which is the positive direction of the c-axis. When the growthdirection is [0001] (e.g., Ga-polarity), a pyramidal or cone-likegeometry is achieved for the GaN nanowires. Conversely, the structurehas N polarity when the growth direction is [0001], which often exhibitsa flat top surface. In the case of GaN wurtzite crystal structures, thepolarity depends critically on the growth kinetics, buffer layers,substrates, and growth systems. As shown in FIG. 8, the n-GaNnanocrystals 104 are formed in a pyramidal or conical shape with theGa-polarity because of the Ga-terminated GaN template on the substrate106. Consequently, this Ga-polar crystal geometry leads to the formationof a cone-shaped semi-polar multi-quantum disk layers in the activeregion.

In FIG. 6B, the nanocrystal array 102 is first planarized by aspin-coating system using a polyimide layer 602, followed by plasmaetching to expose the top surfaces of the nanocrystals 104. In FIG. 6C,a silicon oxide (SiO_(x)) passivation layer 610 is deposited byplasma-enhanced chemical vapor deposition (PECVD), and the active regionarea corresponds to the opening aperture 612 on the SiO_(x) layer. InFIG. 6D, a metal electrode 614 is deposited on the top surfaces of thep-GaN region 114 of the nanocrystals 104 by an e-beam evaporator with ane-beam lithography technique and then annealed. In an embodiment, theelectrode 614 consists of nickel (Ni) and gold (Au). Subsequently, anindium tin oxide (ITO) layer 616 is deposited, to serve as a transparentelectrode and current spreading layer. The ITO layer 616 is alsoannealed. In FIG. 6E, contact layers are deposited on the surroundingtop surface of the ITO layer 616 and n-GaN template 704 to serve as thep-metal and n-metal contacts 618 and 620. In an embodiment, the p-metalcontact 618 consists of Ni and Au, and the n-metal contact 620 consistsof Ti and Au. The fabricated devices with metal contacts can then beannealed.

FIG. 6F shows a cross-section of an embodiment of the NCSEL 600 that isfabricated as just described. In the example of FIG. 6F, the ITO layer616 is light-transmissive, and laser light is emitted vertically in thedirection away from the substrate 106 and out of the NCSEL 600 throughthe ITO layer.

To achieve stable and low resistivity ohmic contact, the thickness of Niand Au in the contact 620 and the annealing process can be optimized sothat, upon annealing, Ni may form NiO_(x) and become nearly transparent.The thin Au layer, however, does partially block the emitted light.However, it is estimated that the transmittance of the Au metal layer isabout 60-70 percent in the green wavelength region, which leads tonon-negligible optical loss and limits the optical power.

In an alternate embodiment, the ITO layer 616 is replaced with alight-reflective layer, and the substrate 106 is light-transmissive.That is, the substrate 106 consists of a light-transmissive materialinstead of, for example, AL₂O₃. In such an embodiment, laser light isemitted vertically in the direction away from the substrate 106,reflected back toward the substrate by the light-reflective layer, andout of the NCSEL through the substrate.

To summarize, disclosed herein is a new generation of surface-emittinglaser diodes utilizing nanocrystals grown from the bottom (substrate)up. Such a unique structure is not fabricated by the conventionaltop-down approach because the active region is predefined by the filmstructure. The presence of a clear threshold, sharp linewidth reduction,distinct far-field emission pattern, and polarized light emissionprovides unambiguous evidence for the achievement of coherent lasingoscillation. Significantly, compared to conventional VCSELs, lasing andsurface emission is achieved without using thick, resistive, and oftenheavily dislocated DBRs. This unique laser concept can be readilyextended to achieve all-epitaxial, DBR-free surface-emitting laserdiodes operating across the entire visible spectrum as well as mid anddeep UV wavelengths, and to realize such lasers on low-cost, large-areawafers (e.g., silicon wafers). Disclosed herein is a new paradigm in thedesign and development of surface-emitting laser diodes, where theperformance is not limited by the availability of DBRs, latticemismatch, and substrate availability.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the present disclosure is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the present disclosure.

Embodiments according to the invention are thus described. While thepresent disclosure has been described in particular embodiments, theinvention should not be construed as limited by such embodiments, butrather construed according to the following claims.

What is claimed is:
 1. A nanowire, comprising: a first semiconductorregion; a second semiconductor region; and a heterostructure disposedbetween and coupled to the first semiconductor region and the secondsemiconductor region; wherein the nanowire is operable for emittingstimulated emission light having a wavelength in a range of 520-560nanometers at a current density that is at least an order of magnitudeless than ten kiloamperes per square centimeter (10 kA/cm²).
 2. Thenanowire of claim 1, wherein the current density is on the order of 0.4kA/cm² and less.
 3. The nanowire of claim 1, wherein the firstsemiconductor region comprises n-doped gallium nitride, and wherein thesecond semiconductor region comprises p-doped gallium nitride.
 4. Thenanowire of claim 1, wherein the heterostructure comprises shell layersand quantum disks, and wherein the shell layers comprise aluminumgallium nitride, and the quantum disks comprise indium gallium nitride.5. The nanowire of claim 1, wherein the heterostructure comprises shelllayers and quantum disks, and wherein the quantum disks are interleavedwith the shell layers.
 6. The nanowire of claim 1, wherein theheterostructure is cone-shaped and is narrower as distance from thesubstrate increases.
 7. The nanowire of claim 1, wherein theheterostructure comprises: a quantum disk disposed in a first semipolarplane of the nanowire; and a shell layer disposed in a second semipolarplane of the nanowire.
 8. The nanowire of claim 1, wherein the nanowirehas a transverse cross-section that is hexagonal.
 9. A device,comprising: a substrate; and a surface-emitting laser coupled to thesubstrate and comprising a nanocrystal array comprising a plurality ofnanowires.
 10. The device of claim 9, wherein the nanocrystal arraycomprises an optical cavity.
 11. The device of claim 9, wherein thenanowires have a photonic crystalline structure comprising an opticalcavity; and wherein, in operation, the photonic crystalline structure isconfigured to form a standing wave in the photonic crystalline structurewithout a dynamic Bragg reflector.
 12. The device of claim 11, wherein areciprocal lattice of the photonic crystalline structure comprises sixequivalent and coupled Brillouin zone gamma points.
 13. The device ofclaim 11, wherein the nanocrystal array exhibits a uniform photonicband-edge resonant effect in operation.
 14. The device of claim 9,wherein the nanowires are operable for emitting stimulated emissionlight having a wavelength in a range of 520-560 nanometers at a currentdensity that is at least an order of magnitude less than ten kiloamperesper square centimeter.
 15. The device of claim 9, wherein the nanowiresare operable for emitting stimulated emission light having a wavelengthin a range selected from the group consisting of: 10-400 nanometers; and635-700 nanometers.
 16. The device of claim 9, wherein each nanowire ofthe plurality of nanowires comprises: an n-type semiconductor region; ap-type semiconductor region; and a heterostructure between the n-typesemiconductor region and the p-type semiconductor region, theheterostructure comprising: a quantum disk disposed in a first semipolarplane of the nanowire; and a shell layer disposed in a second semipolarplane of the nanowire.
 17. The device of claim 9, wherein thenanocrystal array includes multiple rows of the nanowires, wherein eachrow includes multiple nanowires, wherein each nanowire of the pluralityof nanowires has a transverse cross-section that is hexagonal, andwherein the nanowires are arranged in a triangular lattice in the array.18. A monolithic device, comprising: a substrate layer; and asurface-emitting laser diode coupled to the substrate layer andcomprising a nanostructure array comprising a plurality of nanowires,wherein the nanowires are operable for emitting stimulated emissionlight having a wavelength in a range of 520-560 nanometers.
 19. Thedevice of claim 18, further comprising an n-type semiconductor layerdisposed on the substrate layer and comprising a uniform array of holes,wherein each nanowire of the plurality of nanowires extends from arespective hole of the array of holes.
 20. The device of claim 18,wherein a nanowire of the plurality of nanowires comprises: an n-typesemiconductor region; a p-type semiconductor region; and aheterostructure between the n-type semiconductor region and the p-typesemiconductor region, wherein the heterostructure comprises: a quantumdisk disposed in a first semipolar plane of the nanowire; and a shelllayer disposed in a second semipolar plane of the nanowire.
 21. Thedevice of claim 18, wherein the nanowires have uniform diameters,lengths, and shapes and are uniformly spaced within the nanostructurearray.
 22. The device of claim 18, further comprising alight-transmissive layer coupled to the nanostructure array.
 23. Thedevice of claim 18, wherein the substrate layer is light-transmissive.